A light emitting diode (LED), which is a type of semiconductor light source, is a compound semiconductor having a p-n junction structure. The LED refers to a device that emits light by recombination of minority carriers (electrons or holes). The LED is increasingly used for various lighting applications, as well as parts of information processing and communication apparatuses because it has low power consumption and long life span. And the LED may be installed in a limited space, and has strong endurance against vibration. Also, the LED has been manufactured in various types of light emitting devices which are suitable for applications. In particular, in addition to the device emitting light of a single wavelength such as a blue, green or red wavelength, white light emitting devices have been launched in recent years. As the white light emitting devices are applied to products for automobiles and general lighting products, their demands are expected to suddenly increase. There are three representative methods of obtaining white light using the LED.
First, there is a method of obtaining a white color by a combination of three LEDs generating the three primary colors of light such as red, green and blue, of light. In this method, luminous efficiency is improved, but exact control of operating drivers is required to obtain a white color with certainty. Also, products may be increased in size, optical properties and reliability of the products may be affected due to different temperature properties of the three LEDs.
Second, there is a method which uses a blue LED as a light source. In this method, yellow phosphors are excited, and blue light which is not absorbed by the phosphors is combined with yellow light to obtain a white color. In this case, since the yellow phosphors should be excited in the vicinity of 450 nm, there is a limit to the kinds of the phosphors, and color rendition is not good.
Third, there is a method of exciting the three primary color phosphors to obtain a white color using an ultraviolet (UV) LED as a light source. Since this method is used to obtain white light using three wavelengths which are converted by the UV rays, it is excellent in theory in that the type of phosphors are diverse and high color rendition is expected. In practice, however, the method has problems regarding low conversion efficiency of the phosphors, degradation of a sealing material by the UV rays, or eye safety in case of UV leakage.